1. Field of the Invention
The present invention relates to a transmission method and a transmitter using the Ultra Wide-Band (UWB) system for transmission.
2. Description of Related Art
Particular attention has been paid to the UWB system as one of wireless transmission systems. The UWB system realizes transmission using a very wide transmission band of, for example, several gigahertzes and using very short pulses.
A recent trend is to put SS (Spread Spectrum) based wireless LAN (Local Area Network) systems to practical use. There are proposed SS based UWB transmission systems for PAN applications and the like.
The SS systems include the DS (Direct Spread) system. According to this system, the transmission side multiplies an information signal by a random code sequence called a PN (Pseudo Noise) code to spread a dedicated band for transmission. The reception side multiplies the received spread information signal by the PN code to reversely spread the information signal for reproduction.
The UWB transmission system includes two types: DS-UWB and impulse-UWB. The DS-UWB system maximizes spread code speeds of DS information signals. The impulse-UWB system configures an information signal using an impulse signal sequence having a very short cycle of approximately several hundred picoseconds to send and receive the signal sequence.
The DS-UWB can control spectra using PN code speeds, but needs to fast operate logic circuits in units of GHz. The power consumption increases dramatically. On the other hand, the impulse-UWB system can be configured in combination with a pulse generator and a low-speed logic circuit. There is an advantage of decreasing the current consumption. However, the pulse generator makes it difficult to control spectra.
Both systems implement high-speed data transmission by spreading signals to an ultra-high frequency band, e.g., between 3 and 10 GHz for transmission and reception. The dedicated bandwidth is expressed in units of GHz so that a value approximate to 1 results from division of the dedicated bandwidth by a center frequency (e.g., 1 to 10 GHz). The dedicated bandwidth is ultra wide compared to bandwidths normally used for wireless LANs based on the W-CDMA or cdma2000 system, and the SS (Spread Spectrum) or OFDM (Orthogonal Frequency Division Multiplexing) system.
Since the impulse-UWB system uses a very narrow pulse for the impulse signal, a very wide band is used in terms of the frequency spectrum. Consequently, an input information signal merely indicates a power smaller than the noise level in respective frequency domains. Available modulation systems include PPM (Pulse Position Modulation) to represent a code according to a position between pulses, Bi-phase Modulation to represent a code according to a pulse's phase change, and amplitude modulation.
FIG. 10 shows a configuration example of a conventional UWB transceiver. An antenna 11 is connected to an antenna changer 13 via a band-pass filter 12. The antenna changer 13 is connected to reception-related circuits and transmission-related circuits. The antenna changer 13 functions as a selection switch to operate in interlock with transmission and reception timings. The band-pass filter 12 passes signals of transmission bandwidths of several gigahertzes such as 4 to 9 GHz used for the system.
The reception-related circuits connected to the antenna changer 13 include a low noise amplifier 14, 2-system multipliers 15I and 15Q, low pass filters 16I and 16Q, and analog-digital converters 17I and 17Q. The low noise amplifier 14 amplifies an output from the antenna changer 13 for reception. The multipliers 15I and 15Q multiply an output from the low noise amplifier 14 by outputs from pulse generators 25I and 25Q. The low pass filters 16I and 16Q eliminate high frequency components from outputs from the multipliers 15I and 15Q. The analog-digital converters 17I and 17Q sample outputs from the low pass filters 16I and 16Q.
Output pulses from the pulse generator 25I and 25Q are phase-shifted from each other by the specified amount. The analog-digital converter 17I samples I-channel transmission data. The analog-digital converter 17Q samples Q-channel transmission data. Received data for each channel is supplied to the baseband circuit 30 for reception processing. In this example, received data for the I channel is used as is. Received data for the Q channel is used as an error signal.
As transmission-related circuits, the multiplier 26 is supplied with transmission data output from the baseband circuit 30. The transmission data is multiplied by an output from the pulse generator 25I. The transmission data output from the baseband circuit 30 is modulated, e.g., as an NRZ (Non Return to Zero) signal. The multiplier 26 multiplies the transmission data by an output from the pulse generator 25I to generate a bi-phase modulated pulse. This becomes a signal modulated by the so-called BPSK (Binary Phase Shift Keying) system. In order to allow the pulse generator 25I to generate pulses, there is provided a Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO, hereafter simply referred to as an oscillator) 21 to control oscillation frequencies of the oscillator 21 based on an error signal acquired from received data for the Q channel, for example.
An oscillation signal from the oscillator 21 is supplied to a PLL (phase locked loop) circuit 22. A voltage control oscillator 23 constitutes a loop for the PLL circuit 22. An oscillated output from the voltage control oscillator 23 is supplied to the pulse generator 25I to generate a pulse synchronized to the oscillated output from the oscillator 23. A phase shifter 24 supplies a pulse generator 25Q with an output from the oscillator 23 by delaying a specified cyclic phase. This makes it possible to generate a short wavelength pulse synchronized with the oscillated output from the oscillator 23 at a timing delayed from an output pulse of the pulse generator 25I.
A multiplier 26 multiplies an output pulse from the pulse generator 25Q by the transmission data to use the multiplication output as a transmission signal. The transmission signal output from the multiplier 26 is supplied to a power amplifier 27 and is amplified there for transmission. The amplified output is supplied to the band-pass filter 12 via the antenna changer 13. The band-pass filter 12 limits the band to pass only signals for the transmission band. The transmission signal is then transmitted from the antenna 11.
Non-patent document 1 outlines the UWB system.
[Non-patent document 1]
Nikkei Electronics, 11 Mar. 2002, pp. 55–66.
A pulse used for the impulse-UWB system is a signal having the wideband frequency spectrum. The time domain is equivalent to a monocycle waveform expressed by equation 1, for example.
                              V          ⁡                      (            t            )                          =                                            e                                      t              P                                ·          t          ·                      exp            ⁡                          [                                                -                                      1                    2                                                  ·                                                      (                                          t                                              t                        P                                                              )                                    2                                            ]                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In equation 1, tP represents the time from the monocycle waveform center to a peak value. In the case of tP=200 [psec], for example, the time waveform becomes a monocycle waveform generated at its maximum value of ±200 [psec] as shown in FIG. 11. We can confirm that the monocycle waveform's spectrum has the maximum value of approximately 1 [GHz] and the −3 dB bandwidth of approximately 1 [GHz].
We examine generating a single sideband of the monocycle waveform for frequency conversion. The reason is that the UWB system specifies the following two spectrum requirements for transmission pulses.
(1) The US FCC spectrum mask specification, one of UWB specifications, requires that radiation levels be decreased in the bands except 3.1 through 10.6 [GHz].
(2) The band of 4.9 through 5.8 [GHz] contains 5 GHz wireless LANs that should be avoided.
In consideration for these requirements, we can assume to be able to solve the above-mentioned problems of the spectrum in the UWB communication system as follows. That is to say, the spectrum in FIG. 12 is converted into the frequency range, e.g., between 3.1 and 4.9 [GHz] to generate an upper side band spectrum as shown in FIG. 13. There is provided a method of frequency converting the monocycle waveform in the upper side band. The method subtracts a signal obtained as a product of multiplying a Hilbert transformed monocycle waveform in FIG. 14 by a 3.1 [GHz] sine carrier from a signal obtained as a product of multiplying the monocycle waveform by a 3.1 [GHz] cosine carrier.
A pulse waveform in FIG. 15 represents the time waveform resulting from the spectrum in FIG. 12. The envelope's amplitude gradually increases, peaks at the origin, and gradually decreases. Accordingly, it can be understood that the envelope approximates to a triangle. Further, it can be understood that a 6-cycle pulse waveform constitutes major amplitude components.
To solve the above-mentioned problems of the spectrum in the UWB communication system, we can come to a solution generate an N-cycle pulse whose envelope is amplified and is formed as a triangle. For example, the waveform in FIG. 15 has the duration of approximately 2 [nsec]. Arranging this pulse waveform in a series enables the BPSK communication at 500 [Mb/s] by preventing a series of pulse waveforms from overlapping with each other.
To achieve a higher communication rate such as 1 [Gb/s], however, the waveform in FIG. 15 needs to be arranged at a 1 [nsec] interval. Consequently, some waveforms may overlap with each other. When the receiver uses a band-pass filter, it is known that an impulse response of the band-pass filter causes a previous pulse's amplitude to affect the subsequent pulses. This problem is called an inter-symbol interference and should be considered when narrowing the band for improving the frequency utilization.
According to the Nyquist's theorem, a baseband bandwidth of ½T [Hz] is required to transmit pulses at a T [sec] interval without distortion, where 1/T [Hz] is the Nyquist bandwidth. Since the frequency under discussion ranges from 3.1 to 4.9 [GHz], the bandwidth is 1.8 [GHz] and the baseband bandwidth is its half, i.e., 900 [MHz]. It fully ensures the minimum baseband bandwidth of 500 [MHz] to transmit pulses at a T [sec] interval but is 10[%] fall short of the 1 [GHz] Nyquist bandwidth.
Nyquist showed that the Nyquist filter should be used to satisfy the condition of no distortion below the Nyquist bandwidth. However, it is difficult to create a 1 [GHz] baseband digital filter. The reason is that creating the intended digital filter requires, e.g., an 8-bit D/A converter operating at least at a sampling frequency of approximately 4 [GHz]. Presently, there is a marketed example as a standalone unit that uses four D/A converters at 1.25 [Gsamples/sec] to acquire 5 [Gsamples/sec] Though such product is available on the current technological level, the design is unfavorable from the viewpoint of the cost effectiveness between the power consumption and installation costs when the UWB communication is applied to the consumer equipment.
An alternative to the baseband digital filter may be a band-pass filter (BPF) for high frequency bands. FIG. 16 shows an impulse response when a 5-polar Butterworth filter is used for the BPF having the 4 [GHz] center frequency and the 1.8 [GHz]band. As seen from the impulse response in FIG. 16, its main wave in the vicinity of 0.8 [nsec] indicates that the BPF is subject to a delay time of 0.8 [nsec]. However, there is also generated a swell as large as one third of the main wave in the vicinity of 1.8 [nsec]. In this manner, an inter-symbol interference occurs due to the impulse response when a non-Nyquist filter is used.